amavis ldap trouble in process_request do_search

[frank.kaiser at ewetel.net]

I have some trouble with amavis ldap postfix ldap on CentOS 6.4  
Sometimes I found:

amavis[1170]: (01170-02) (!)lookup_ldap: do_search: failed:  
LDAP_OPERATIONS_ERROR
  amavis[1170]: (01170-02) (!!)TROUBLE in process_request: do_search:  
failed: LDAP_OPERATIONS_ERROR at (eval 113) line 538.
amavis[1170]: (01170-02) (!)Requesting process rundown after fatal error

In amavis log.

In this case no entry can found ldap.log on ldap host.

It happens not on every mail scan but only every 4th or 5th mail. Mail  
turns temporary to queue with:

relay=127.0.0.1[127.0.0.1]:10024, delay=0.21, delays=0.12/0.01/0/0.08,  
dsn=4.3.2, status=deferred (host 127.0.0.1[127.0.0.1] said: 421 4.3.2  
Service shutting down, closing channel (in reply to RCPT TO command))

Installed:
amavis 2.8.0-4.el6
net-server 0.99-1.el6

any ideas